The present invention relates to a resist pattern forming method used in manufacturing a semiconductor device and the like. In this specification, a resist means a photoresist.
In a photolithography process, a super-resolution exposure technique using a phase-shifting mask and the like must be applied to form a fine pattern with a width smaller than the wavelength of exposure light.
A Levenson phase-shifting mask comprises, e.g., a transparent substrate, light-shielding members arranged parallel to each other on the transparent substrate at predetermined intervals, and phase shifters arranged every other interval between adjacent light-shielding members. The Levenson phase-shifting mask is formed to attain a phase difference of 180.degree. between exposure light transmitted through a transmitting portion not having a phase shifter, and exposure light transmitted through an adjacent transmitting portion having a phase shifter.
Of phase-shifting masks, the Levenson phase-shifting mask can form a periodic, high-density resist pattern at a higher resolution. With the use of the Levenson phase-shifting mask, a larger depth of focus (DOF) can be obtained.
However, when, e.g., the core of a semiconductor device is to be formed using the Levenson phase-shifting mask, transmitting portions having phase shifters or transmitting portions not having phase shifters may be partially adjacent to each other because the core pattern is complicated. No phase difference can be formed at the location where transmitting portions of the same type are adjacent to each other. At this location, the resolution cannot be increased. For this reason, the core pattern is formed with a relatively large size, whereas the cell pattern is generally formed with a size as small as possible.
In general, exposure light transmitted through a photomask diffracts or interferes, varying in intensity and the like. That is, the intensity of exposure light incident on a resist film is not uniform but varies at the entire irradiated portion. The intensity distribution (image intensity distribution) depends on diffraction and interference conditions, i.e., the pattern size and pattern density (duty ratio) of a light-transmitting portion formed on the photomask. Therefore, optimum conditions, such as the optimum exposure dose, for exposing the resist film formed on the substrate depend on the size and the density. Accordingly, it is difficult to expose both core and cell portions under optimum conditions in simultaneously forming them.
This problem occurs not only in the use of the Levenson phase-shifting mask but also in the use of a general photomask. Therefore, demands arise for a larger margin for exposure conditions such as the exposure dose in order to form a pattern at a higher precision in manufacturing a semiconductor device.
Prior to the exposure, a resist film must be formed on a substrate. A resist film is formed on the entire coating surface of a substrate by a spin coating method as a conventional resist coating method. The resist film is also applied on an alignment mark used to align the substrate, a photomask, and the like in exposure. As a result, the alignment precision decreases, and no resist pattern can be formed at a high precision.
In addition, since the resist film is formed on a region, such as the alignment mark, where no resist film need be formed, a larger amount of resist is required, resulting in high cost.